Automated friction clutches in a drivetrain of a motor vehicle between an internal combustion engine and a transmission have long been known. Here, instead of the foot of a driver, a clutch actuator controlled by a clutch device moves an actuating element, for example a clutch lever, a diaphragm spring, a lever spring or the like, along an actuation path. Assigned to the actuation path is a torque characteristic curve which can be adapted, for example, to external conditions such as clutch temperature, the frictional properties of the clutch linings, operation time and the like, and which may be calibrated by means of a contact point on the actuation path. For example, depending on the operating situation of the friction clutch desired by the driver or resulting from a transmission controller, a target clutch torque or a value connected therewith is ascertained by a control unit and issued as a control variable to adjust an actuation path of the clutch actuator corresponding to the target clutch torque. Depending on the design of the clutch actuator, with electric drive of the clutch actuator this variable may be an electrical variable such as voltage, current or pulse width of a supply voltage, or a pressure, a volume flow or the like in the case of a hydraulically or pneumatically operated clutch actuator. The adjustment of the actuation path may be monitored or regulated by means of relative and/or absolute distance sensors.
In friction clutches of this sort, due to the geometric properties and manufacturing tolerances not corresponding to the ideal state, for example angle and axis offsets between the friction elements of the friction clutch resulting in uneven frictional engagement, so-called chatter vibrations may occur, in which case there is superimposed on the target clutch torque adjusted on the basis of the predetermined target clutch torque a disturbance clutch torque overlaid with a predetermined amplitude and frequency, which may result in disruptions of comfort of the motor vehicle and increased wear. To reduce such chatter vibrations, a method is known, for example from unpublished German patent application No. 10 2012 217 132.0, in which the frequency, amplitude and phase of a vibration superimposed on the transmission input signal is ascertained. In this method, depending on the frequency ascertained, an amplified and phase-shifted signal of the same frequency is generated and modulated as a control signal onto the target clutch torque, so as to eliminate oscillations of the transmission input signal. When multiple frequency components occur with comparable amplitude in the ascertained range, this compensation can result in behavior that is difficult to track. If, in addition, a phase jump occurs in the transmission input signal, frequency is difficult to determine; the same is true in the case of greatly changing amplitude or frequency, since modulations of amplitude, phase and frequency are interconnected.
In unpublished German patent application no. 10 2013 204 698.7, a method is disclosed for damping drivetrain vibrations in a general form by eliminating a resonant frequency. This method provides only a limited possibility of reducing the chatter vibrations due to geometry. It is true that known excitation frequencies can be varied by adjusting filter parameters so as to perform an optimization for this excitation, but other excitation frequencies can be intensified thereby; for example, a chattering caused by geometry in a different frequency may be promoted.